Heavy or settlement-sensitive facilities that are located in areas containing soft or weak soils are often supported on deep foundations, consisting of driven piles or drilled concrete piers. The deep foundations are designed to transfer the structure loads through the soft soils to more competent soil strata.
In recent years, aggregate piers have been increasingly used to support structures located in areas containing soft soils. The piers are designed to reinforce and strengthen the soft layer and minimize resulting settlements. The piers are constructed using a variety of methods including the drilling and tamping method described in U.S. Pat. Nos. 5,249,892 and 6,354,766 (“short aggregate piers”), the driven mandrel method described in U.S. Pat. No. 6,425,713 (“Lateral Displacement Pier), the tamper head driven mandrel method known as the “Impact Pier” (U.S. Patent Pending), and the driven tapered mandrel method (U.S. Patent Pending).
The Short Aggregate Pier method (U.S. Pat. Nos. 5,249,892 and 6,354,766), which includes drilling or excavating a cavity, is an effective foundation solution when installed in cohesive soils where the sidewall stability of the hole is easily maintained. The method generally consists of: a) drilling a generally cylindrical cavity or hole in the foundation soil; b) compacting the soil at the bottom of the cavity; c) installing a relatively thin lift of aggregate into the cavity; d) tamping the aggregate lift with a specially designed beveled tamper head; and e) repeating the process to form an aggregate pier generally extending to the ground surface. Fundamental to the process is the application of sufficient energy to the beveled tamper head such that the process builds up lateral stresses within the matrix soil up the sides of the cavity during the sequential tamping. This lateral stress build up is important because it decreases the compressibility of the matrix soils and allows applied loads to be efficiently transferred to the matrix soils during pier loading.
The tamper head disclosed in U.S. Pat. No. 5,249,892 is flat on the bottom and has beveled sides. In commercial practice, the flat bottom portion of the beveled tamper makes up approximately 70% of the tamper cross-sectional area, while the beveled portion comprises only about 30% of the tamper cross-sectional area. Thus, approximately 70% of the tamper cross-sectional area is devoted to the downward compaction of each lift of aggregate (done by the flat bottom portion) and only 30% of the tamper cross-sectional area is devoted to increasing the lateral pressure in the matrix soil (done by the beveled portion). As so constructed, the beveled tamper head of the Short Aggregate Pier method is used most often to compact 12 inch lifts of aggregate in 30 inch diameter holes.
The tamper head driven mandrel method (“Impact Pier” method) is a displacement form of the Short Aggregate Pier method. The “Impact Pier” method consists of driving a hollow pipe (mandrel) into the ground without the need for drilling. The pipe is fitted with a tamper head at the bottom which has a greater diameter than the pipe and which has a flat bottom and beveled sides. The mandrel is driven to the design bottom of pier elevation, filled with aggregate and then lifted, allowing the aggregate to flow out of the pipe and into the cavity created by withdrawing the mandrel. The tamper head is then driven back down into the aggregate to compact the aggregate. The flat bottom shape of the tamper head compacts the aggregate; the beveled sides force the aggregate into the sidewalls of the hole thereby increasing the lateral stresses in the surrounding ground.
The driven tapered mandrel method is another means of creating an aggregate pier with a displacement mandrel. In this case, the shape of the mandrel is a truncated cone, larger at the top than at the bottom, with a taper angle of about 1 to about 5 degrees from vertical. The mandrel is driven into the ground, causing the matrix soil to displace downwardly and laterally during driving. After reaching the design bottom of the pier elevation, the mandrel is withdrawn, leaving a cone shaped cavity in the ground. The conical shape of the mandrel allows for temporary stability of the sidewalls of the hole such that aggregate may be introduced into the cavity from the ground surface. After placing a lift of aggregate, the mandrel is re-driven downward into the aggregate to compact the aggregate and force it sideways into the sidewalls of the hole. Sometimes, a larger mandrel is used to compact the aggregate near the top of the pier.